Large heat exchangers, particularly steam generators in pressurized water reactor (PWR) nuclear power plants, typically contain a multiplicity of tubes secured at their ends to a tube sheet. The tube sheet serves not only to secure the tubes in place, but also to separate the shell side fluid outside the tubes, from the fluid passing through the tubes.
In the vertically oriented PWR steam generators, crud or sludge deposits accumulate on the tube sheet and in some cases produce deterioration of the tube wall at an elevation immediately above the tube sheet. Since it is virtually impossible to replace individual tubes in a nuclear steam generator, one solution has been install a sleeve within each such deteriorated tube.
The sleeve must provide a substantially leak tight seal whereby the deterioration portion of the tube is isolated from the fluid flow within the tube. In nuclear steam generators, even if the deterioration continues and the tube is perforated, the radioactive fluid from the tube side must not have a flow path to the shell side.
Although seemingly a simple matter, proper and effective sleeving of steam generator tubes presents several difficult problems. Repair work to the steam generator, such as sleeve installation, is performed in a highly radioactive environment, even though the steam generator is first drained. Thus, it is desirable that each sleeve be installed in a minimum of time, and preferably be adapted for remote, automated installation. Some tube sleeving techniques deform the sleeve into the surrounding tube, but this is much easier at the elevation of the tube sheet where the tube has a rigid backup. At elevations above the tube sheet, the tube has no backup and the amount of sleeve deformation into the tube must be carefully controlled and limitied to avoid overstressing the tube. Concern over excess deformation greatly limits the ability to confidently effect a tight seal between the sleeve and tube by merely deforming one against another.
Furthermore, the sleeve and tube expand and contract according to the operating history of the power plant, yet the seals therebetween must remain effective. Also, the sleeve must not provide sites where crud or other solids may accumulate and induce further deterioration of the tube or sleeve. Finally, the sleeve must perform its desired function over the remaining life of the steam generator, which may typically be in excess of thirty years.